rpmsg: Change naming of mediatek rpmsg property

Change from "mtk,rpmsg-name" to "mediatek,rpmsg-name" to sync with the
vendor name defined in vendor-prefixes.yaml.

Signed-off-by: Tinghan Shen <tinghan.shen@mediatek.com>
Link: https://lore.kernel.org/r/20210924033935.2127-6-tinghan.shen@mediatek.com


[Fixed capital letter in title]
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>